Shielded magnetic core



June 3, 1969 R BERG ET AL 3,448,421

SHIELDED MAGNETIC CORE Original Filed June 7, 1962 s 9 g 7 K 2 6 7 vi NF x 513:

FIG. I g, 3

55 7 2 3 v 4 M W 0:.

FIG. 2

7 l7 rL I \Q\\V 2 3 1 I5 ML FIG. 5

INVENTCRS.

BRADFORD HOWLAND BY ROBERT S. BERG United States Patent US. Cl. 336-84 8Claims ABSTRACT OF THE DISCLOSURE A low leakage flux magnetic coresuitable for use as the core of an inductor or transformer is described.A ferrite core has deposited on it two electrically conductive layerswhich are separated by an electrically insulating layer of plastic. Eachconductive layer has a continuous insulating gap which is parallel tothe flux path in the core with the gaps in each layer being on oppositesides of the core from each other.

The invention herein described was made in the course of work performedunder a contract with the Electronic Systems Division, Air Force SystemsCommand.

This application is a division of our co-pending application No.200,748, filed June 7, 1962, now Patent No. 3,336,662, for Shielding aMagnetic Core.

This invention relates to a low leakage-inductance transformer and inparticular to a high frequency transformer with a magnetic core shieldedfrom the windings by a chemically and electrically depositedelectrostatic shield.

A transformer capable of operating over a frequency band ofapproximately 1 to- 100 megacycles finds many applications in electricalcircuits which cover this frequency range. Satisfactory frequencyresponse has been obtained by transformers constructed in accordancewith the invention of this application in hybrid junction or magic Tcircuits where a turns ratio approximating 2:1 is required. Distributedtransformer techniques, while providing the necessary bandwidth were notused because of the difliculty in getting the required turns ratio. Theuse of the eddy-current shield of the present invention reduces theleakage inductance by an order of magnitude with consequent improvementin the high frequency response of a transformer Wound by conventionaltechniques.

Prior art transformers have used eddy current shielding to reduceleakage inductance. One type of eddy current shielding which is used ona toroidal core consists of a copper screen having a circumferential gapalong the inner surface of the toroid. Such a shield is discussed in anarticle in the Wireless Engineer, June, 1947, pp. 175-176 where it isstated that the windings of the transformer may be placed outside thescreen. The general outline of such a shield is similar to that shown inFIGURE 3 of this application. Another type of shield which is describedin the General Radio Experimenter, vol. XXX, No. 11, April 1956,consists of two copper toroidal cups of different diameters which slipover a toroidal magnetic core to form a shield similar to that of FIGURE5 of this application. Neither of these techniques are suitable forapplication to magnetic cores especially suited for high frequencytransformers. These cores are physically of small dimensions sincetransformers in general improve in high frequency performance as thesize decreases. Since space for the transformer windings is at apremium, further reduction of this winding space by eddy currentshielding must be kept to a minimum. The copper screen shield is3,448,421 Patented June 3, 1969 bulky and not suitable for small cores.The copper cups are diflicult to fabricate in small sizes and with thethin walls required to avoid occupying a large portion of the centerhole winding space of the toroidal core. In addition, copper cups arerestricted to use with toroidal cores since fabrication of the cups forother shapes by machining techniques would be diflicult.

Accordingly, it is an object of this invention to provide a means foreddy current shielding of small magnetic cores which is easy tofabricate and which reduces the available winding space by a negligibledegree.

It is a further object of this invention to provide an eddy currentshield which may be easily fabricated on cores of shape other thantoroidal.

These objects are obtained in the present invention by chemical andelectrical deposition of a conducting surface over the entire core andby removing said conductor along a circumferential path to avoid a shortcircuited turn. A second conducting surface applied on a nonconductivecoating on said first conducting surface and similarly modified to avoida short circuited turn will provide additional reduction in leakageinductance.

The novel features of the invention together with further objects andadvantages thereof will become apparent from the following descriptiontaken in connection with the accompanying drawings wherein:

FIGURE 1 shows in cross section a single layer conductive material eddycurrent shield on a toroidal core.

FIGURE 2 shows in cross section a two layer shield.

FIGURE 3 shows in cross section another single layer shield with a gapon a circumference of the core.

FIGURE 4 shows in cross section the preferred embodiment of the twolayer shield.

FIGURE 5 shows in cross section a multiaperture core with windings onthe legs thereof.

FIGURES 1 through 4 show in cross section a toroidal core 1 on which acopper plated shield 2 has been deposited. Copper shield 2 serves as theeddy current shield which is of a shape inconvenient for directmechanical fabrication and is most easily formed by electroplatingdirectly on the core. The purpose of shield 2 is to provide an eddycurrent barrier to a changing magnetic flux with a component normal toshield 2. This barrier prevents flux from leaving core 1 and loopingaround either coil 6 or 7 before reentering the core 1, therebyproducing a leakage inductance in whichever coil is looped. It isapparent that shield 2 cannot be perfectly effective since it must beinterrupted along some circumference to avoid a shorted turn. One way ofeffecting this interruption is shown in FIGURE 1 where a counter-sinkhas been used to remove the plating at the inside corner 5 of thecore 1. Care must be exercised in controlling the amount of materialremoved from corner 5 lest too little material removal causes aninadvertent short circuit, while too much removal will cause anexcessive amount of flux to escape from the core 1 through gap 9,thereby increasing the leakage inductance of either or both coils 6 and7. Another difficulty with the removal of material from a corner of thecore is that the core must be sufliciently uniform in inside diameterand thickness that material is removed uniformly from the corner 5 ofthe core 1.

FIGURE 2 is a cross section of a toroidal magnetic core 1 having a twolayer shield consisting of shields 2 and 4 separated by a nonconductivematerial 3. The short circuited turn produced by shield 4 is interruptedby using a countersink to remove enough of the shield at corner 8 toform gap 10 in the same manner as the gap 9 at corner 5. In FIGURE 2,corners 5 and 8 are chosen as the inside corners of core 1. To reduceleakage flux to a minimum, the gaps 9 and 10 should be at oppositecorners although for convenience of fabrication, both inside or bothoutside corners may be used.

FIGURE 3 shows a core 1 in which the copper shield 2 is interrupted by acircumferential cut 11 made by a fine slitting saw. The difiiculty ofcontrolling the depth of the saw cut 11 makes this embodiment of theinvention somewhat less desirable than FIGURE 1 for a two layer shieldconfiguration. The width of the saw cut 11 should narrow to minimizeflux leakage but for small sized cores, available saws make a wider cutthan is desirable.

FIGURES l and 3 are about equally effective, giving a four foldreduction of leakage inductance for the case of oppositely disposedprimary 6 and secondary 7 windings on a high permeabiilty toroidal core1 as compared to the case where no shield 2 is used.

FIGURE 4 shows in cross section the preferred embodiment of thisinvention. A toroidal core 1 with interlocking separated shields 2 and 4is shown. The magnetic reluctance of the flux leakage path in FIGURE 4is considerably greater than in FIGURES 1 and 3. An eightfold reductionof leakage inductance has been obtained with the construction of FIGURE4 as compared to a core with no shielding. The construction of FIGURE 4has the further advantage that no precision machining operations arerequired, and there is little possibility of a short circuit causing ashorted turn.

Magnetic core 1 is a ferrite core suitable for operation at highfrequencies. Typically, successful transformers have been constructedusing ferrites manufactured by the General Ceramics Company in toroidalcore size CF-l02 (approximately OD. x /6" I.D. x /s), tumbled to removesharp edges. The ferrites Q-l and Q-2 have high bulk resistivity and maybe copper plated directly; type H is of lower resistivity, and for bestresults is coated with a thin film (3-5 mils) of a nonconductive plasticof good dielectric properties before copper plating. An acrylic typeplastic coating has been found to work satisfactorily. The plastic maybe applied by any process suited to the particular plastic used whichwill produce a thin, relatively uniform coating free of pin holes andpreferably free of high spots. Spraying of this plastic has been foundto produce satisfactory results.

The copper plating shields 2 and 4 are applied by a chemical depositiontechnique followed by conventional electroplating to the desiredthickness, 4-5 mils in the case of the shield 2 of FIGURES 1 and 3 and2-3 mils for the shields 2 and 4 of FIGURE 4. A suitable depositiontechnique is described in the book Metallizing of Plastics by Narcus,pp. 14-39, Reinhold Publishing Corp., 1960.

In the construction of the embodiment of FIGURE 4, the first copperplating shield 2 is applied over a plastic coating (not shown) ifrequired by the core 1 resistivity, otherwise directly on the core. Theshield 2 is removed from surface 12 of core 1 by rubbing surface 12 onfine emery paper, being careful to remove sufficient copper plating toavoid a short circuited turn. Next, a layer of -7 mils of plasticcoating 3 is applied as described previously. This is followed by asecond copper plating of 2-3 mils thickness over the entire plasticcoating 3. The plating is removed from surface 13, which is opposite tosurface 12, by rubbing surface 13 on fine emery paper. Care should beexercised in this last operation to remove sufiicientplating to avoid ashort circuited turn while not removing so much material that platingshield 2 is removed also. Windings 6 and 7 may then be wound on thecompleted core assembly to produce a transformer.

Although the embodiment shown in FIGURE 4 shows a core 1 of a toroidalform, the same technique for shielding has been successfully applied totransformers where a three-legged core has been used, shown in crosssection in FIGURE 5. The third leg 14 of the core 1 is in the same planeas the legs 15, 16 of core 1 and of the same thickness. Thus, the copperplating may be removed from the faces 12 and 13 in the same manner asdescribed for the toroidal core of FIGURE 4. The

4 windings 6, 7, and 17, insulated from plating 4, are wound on legs 14,15 and 16 to produce a transformer having low leakage inductance andgood high frequency performance. The core of FIGURE 5 is approximatelytwice the size of the toroidal cores of FIGURES 1 through 4, but withabout the same thickness.

Transformer construction with the cores of FIGURES 1 through 5 consistsof winding the core 1 with Teflon or plastic insulated wires 6, 7 (and17 in FIGURE 5) of thickness chosen to give the correct windingimpedance. For most applications of these transformers, the outerplating shield 4, if a two layer shield is used, will be grounded tominimize electrostatic coupling between the Windings 6, 7 (and 17 inFIGURE 5). The inner shield 2 of FIGURES 2, 4 and 5 being imperfectly exposed to the windings and having large capacitance to the outer shield4, need not be grounded.

In order to avoid severe loss of Q with certain ferrites, such as thetypes Q-l and Q-Z ferrites, following the plating and interruptingoperations, it was found desirable to dry the cores for a day at roomtemperature in a vacuum dessicator after each plating and interruptingoperation. It is believed that the Q of the ferrite cores was reducedbecause of moisture absorption which caused a large decrease in the highfrequency bulk resistivity of these porous ferrite materials.

Transformers for wide bandwidth and high frequencies generally requirevery few turns and may be handwound to a variety of specificationswithout difficulty. Several single-layer windings spaced above agrounded, shielded core will exhibit effective electrosatic isolationtogether with close magnetic coupling. It is seen that the constructionof this invention relegates most of the difficulties to the preparationof the cores, of which only a few types and sizes would normally berequired.

Table I includes the performance specifications of a transformerintended for use in a balanced mixer circuit.

Freq. response (3 db down) 0.3 to 135 mc.

While there has been shown and described what is considered to be thepreferred embodiment of the present invention, it will be obvious tothose skilled in the art that various changes and modifications may bemade therein without departing from the invention as defined in theappended claims.

What is claimed is:

1. A low leakage flux magetic circuit comprising a core having acontinuous flux path, a first electrically conductive non-magnetic filmin complete encompassing contact with said core except for a continuousgap parallel to the flux path in said core, a layer of electricallyinsulating plastic in contact with said film and said insulating gap, asecond electrically conductive nonmagnetic film in complete encompassingcontact with said plastic layer except for a continuous gap parallel tosaid flux path in. said core, the gap in said second film being on theopposite side of said core from the gap in said first film.

2. The magnetic circuit of claim 1 wherein each continuous gap lies in asubstantially planar surface.

3. The magnetic circuit of claim 1 wherein said conducting films are afew thousandths of an inch thick.

4. The magnetic circuit of claim 1 wherein said core comprises a ferritecompletely encompassed by an electrically non-conductive plastic.

5. The magnetic circuit of claim 1 wherein said core is amulti-apertured core having two or more continuous flux paths withinsaid bore, said core having two planar diametric surfaces on oppositesides of said core, each of which contact all flux paths, the gap insaid second film being in the planar surface on the opposite side ofsaid core from the gap in said first film.

6. The apparatus of claim 1 comprising in addition a plurality ofelectrical conductors wound around said core on said second film toprovide transformer windings having low leakage inductance.

7. The apparatus of claim 1 wherein said core is rectangular incross-section transverse to the flux path and said insulating gaps haveWidths which are as wide as the side of the rectangle at which the gapexists.

References Cited UNITED STATES PATENTS 2,724,108 11/1955 Hayes et a1.33684 XR 3,032,729 5/1962 Fluegel 336-84 3,041,561 6/1962 Hannon et a1.336-229 XR 3,149,296 9/1964 COX 336-229 XR LEWIS H. MYERS, PrimaryExaminer. T. J. KOZMA, Assistant Examiner.

U.S. Cl. X.R. 336-229

